An Orbit Fit to Likely Hermus Stream Stars

We selected blue horizontal branch (BHB) stars within the expected distance range and sky position of the Hermus Stream from Data Release 10 of the Sloan Digital Sky Survey. We identify a moving group of $19$ BHB stars that are concentrated within two degrees of the Hermus Stream, between $10$ and $14$ kpc from the Sun. The concentration in velocity is inconsistent with a Gaussian distribution with 98% confidence (2.33 sigma).The stars in the moving group have line-of-sight velocities of v$_{\rm gsr} \sim 50$ km s$^{-1}$, a velocity dispersion of $\sigma_v\lesssim11$ km s$^{-1}$, a line-of-sight depth of $\sim 1$ kpc, and a metallicity of [Fe/H] $=-2.1 \pm 0.4$. The best-fit orbit has a perigalacticon of $\sim4$ kpc, apogalacticon of $\sim 17$ kpc, orbital period of $\sim247$ Myr, eccentricity $e = 0.62$, and inclination $i \sim75^\circ$ from $b=90^\circ$. The BHB stars in the stream are estimated to be $12$ Gyr old. An N-body simulation of a mass-follows-light ultrafaint dwarf galaxy with mass $10^6M_{\odot}$ and radius $40$ pc is consistent with the observed properties. The properties of the identified moving group of 19 BHB stars are close enough to those of the Hermus Stream (which is traced predominantly in turnoff stars) that we find it likely that they are associated. If that is the case, then our orbit fit would imply that there is no relationship between the Hermus and Phoenix streams, as previously proposed.
Comment: 18 pages, 7 figures, Published in MNRAS